dart2js: Add --fast-startup flag.

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index 24a058f317f0a1ccae0cf8995a05b00694614781..72c40abe80c553f6d041ec29957063eda479a12a 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -317,6 +317,7 @@ Future<api.CompilationResult> compile(List<String> argv) {
     new OptionHandler('--library-root=.+', setLibraryRoot),
     new OptionHandler('--out=.+|-o.*', setOutput, multipleArguments: true),
     new OptionHandler('--allow-mock-compilation', passThrough),
+    new OptionHandler('--fast-startup', passThrough),
     new OptionHandler('--minify|-m', implyCompilation),
     new OptionHandler('--preserve-uris', passThrough),
     new OptionHandler('--force-strip=.*', setStrip),